In any given size reduction application, the specific properties of the material being processed play a key role in how finished particle size reduction is achieved. Hardness, brittleness, moisture content, oil content, etc. are all considered when determining not only the appropriate style of size reduction equipment, but also the configuration of the equipment's internal components.

70% Control

In the majority of hammer mill applications, the key factor determining finished particle size is the the screen. Any material that enters the grinding chamber must be reduced to a size small enough to pass through the screen that covers the mill's discharge opening. Because of this the screen size provides 70% of the control over the finished particle size.

Size reduction takes place when material is fed into a hammer mill's grinding chamber and it is repeatedly struck by flailing ganged hammers that are attached to a rotor spinning at very high speed. A combination of hammer blows, collision with the walls of the grinding chamber, and particle on particle impact reduce the material until it is able to pass through the screen.

Sizing up Screens

Screens and bar grates are constructed from steel and are available with perforations (screens) or spaces (bar grates) in a broad range of sizes. Screen size is determined by the size of the openings in the screen, and is described in the following units of measure: inches, millimeters, microns (one millionth of a meter), and US mesh (the number of wires running east/west and north/south in one square inch of screen).

The appropriate screen size is determined by the desired finished particle size, and the properties of the material being processed. That is, characteristics such as friability and moisture content have an effect of the manner in which a material will break down. As a result, using the same screen to process materials of different properties will result in a range of different finished particle sizes.

For example:

This variation is called particle size distribution, and it is based on the the individual properties of the materials being processed.

In this example: Glass is very friable, and will shatter very easily upon impact. In comparison, green wood chips are a fibrous material with a moisture content of up to 50%, which both effect the ease with which they are reduced. Finally, computer hard drives are very hard and comprised mostly of metals, making them comparatively hard to process and unlikely to breakdown beyond the screen or bar grate size.

The Force Factor

But screen size only accounts for 70% of what determines the finished particle size. The reamining 30% is attributed to the force of the impact on the material being processing. In the case of hammer mills, force is determined by rotor speed, and the size and number of hammers.

Let's take a closer look, this time using the example of a drinking glass:

Rotor Speed: Slowly tap the glass with a hammer and it will break into perhaps 3 to 4 large pieces. Conversely, if you hit it with the same hammer at a rapid speed, it will break into many more, much smaller pieces.

Hammer Size: Strike a water glass with a butter knife, and it will break into a few large pieces. Strike the same glass with a sledge hammer, and it will shatter into 1000+ pieces.

In short:

Summary

Finished particle size is determined by a combination of screen size, rotor speed, and the size and number of hammers. Material must remain in the grinding chamber until it is able to pass through the screen covering the hammer mill's discharge opening. Optimal screen size is determined by the desired finished particle size, and the properties of the material being processed.

In earlier posts we discussed how the simplicity of hammer mill technology lends itself well to many applications. On occasion though we encounter an application where the hammer mill is not only effective, it far exceeds the industry standard. E-scrap processing is one of those areas.

Traditionally, whether the goal is data destruction, recycling of materials - or both, e-scrap has been processed in large industrial shredders. Shredders process the e-scrap by using a shearing action that cuts the material as it passes through claw-like knives. While this is method is effective, drawbacks include frequent maintenance to replace the shredder knives, and the costly price point of the standard industrial shredder.

Hammer Mill 101

Hammer mills process material with a pulverizing action. Rectangular steel hammers are attached to a shaft inside of a steel grinding chamber. As the shaft spins, the hammers flail out, bringing them into contact with the material being processed. A combination of hammer blows, particle on particle impact, and contact with the mill interior, work in unison to reduce the material until it is able to pass through the screen or heavy bar grate covering the mill's discharge opening.

A Better Mousetrap

Now let's look at that process as it relates to e-scrap processing, to see how it is more efficient and effective:

1.Flexibility - The simplicity of the hammer mill design offers great flexibility, making it suitable for processing a wide variety of e-scrap materials down to the exact desired finished particle size. Mill size, hammer style and configuration, and screen size are determined based on the processor's production goals. In fact, multiple types of e-scrap media can be processed in the same hammer mill, without reconfiguration.

2.Complete Data Destruction - As it is pulverized, the e-scrap material is continuously scraped and scoured by the turbulent action within the mill. As a result, all retrievable data is destroyed.

3.Liberation of Metals and Other Materials - The second benefit of the pulverizing process is that the hammering action breaks the e-scrap apart, liberating the metals and various other materials. Once broken apart, all pieces remain in the grinding chamber and continue to be pulverized until they are able to pass through the screen covering the mill's discharge opening.

4. Low Cost, High Production - Hammer mills are available in a range of sizes, and are highly customizable based on the material(s) being processed and the user's production goals. A mid-sized hammer mill configured for computer hard drives can process up to 2500 units per hour. A shredder with the same throughput capacity would cost roughly four times that of the hammer mill.

5. Minimal Maintenance - The knives of the shredder dull quickly, and in many cases must be changed monthly. In comparison, one set of 4-way reversible hammers will, on average, last up to one year.

Material is fed in through the top of the mill. Once in the grinding chamber, it is reduced by a combination of repeated hammer blows, particle on particle impact, and contact with the walls of the mill. The material will remain in the grinding chamber until it is reduced to a size that is able to pass through the screen covering the mill's discharge opening.

The simplicity of this design makes it a very versatile hammer mill, one that can be adapted to suit a wide variety of materials, such as:

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Similar, Yet Different

The same hammer mill for fishmeal and coal? Well, yes and no. The basic framework of the mill is the same. However, the configuration of the variable components is how they differ. That determination is based on the following criteria:

Hammer size and style - Number of hammers, size, style and metallurgy.

Screens or bar grates - Style and thickness of screen or bar grates, and size of openings.

Choice of proper RPM

It's Optional

Finally, once the the mill is configured, the last determination is whether or not any optional peripheral equipment is needed. For this, the following questions must be answered:

How will the material be fed into the mill? By hand, auger, or belt conveyor?

How will the material be taken from the mill? Heavy materials such as stone or metal may evacuate via gravity, while light or low density materials will require pneumatic suction.

Is dust a concern?

Answers to these questions will help to determine the best types of optional equipment such as belt conveyors, augers, rotary feeders, and dust collection, as well as the most efficient design of the infeed and discharge chutes.

And that's about it...If you would like more information, please click below to download our free eBook!

Whether it is space constraints or small production, a laboratory scale hammer mill is often the right choice for size reduction. Often used for testing and batch sampling, common installations include: colleges and universities, government testing facilities, and pilot plants.

Below are answers to the top five questions we receive about our laboratory scale hammer mills:

1. Can my material be processed in a lab scale hammer mill?

The laboratory scale hammer mill has the same full range of capabilities as larger industrial hammer mills. Applications include: chemicals, glass, food waste, biomass, lathe turnings, metal powders and much more. Material infeed size and production goals are the ultimate determining factors on whether or not a lab scale hammer mill is suitable for your operation.

2. How durable is the lab scale hammer mill?

The lab scale mills are constructed based on customer specification. The options include carbon steel and 304 or 316 stainless steel. Stainless steel models feature all stainless steel contact surfaces. Replaceable internal liner plates are also an option, The liner plates protect the grinding chamber from wear that results from processing abrasive materials.

3. Can a lab scale hammer mill produce multiple particle sizes?

Yes. In an earlier post we discussed the factors determining particle size. In short: the screen size, shaft speed and hammer configuration selected give you the ability to steer the particle size in any direction you require. This principle holds true with the lab scale hammer mill as well.

Hammer configuration is determined by the material and processing goals, and is generally not changed once mill construction is complete. However, shaft speed and screen size are quite flexible. Screens can be purchased in multiple sizes and are easily interchanged. The smaller the opendings in the screen, the finer the product that is produced. An optional Variable Frequency Drive (VFD) control allows the operator the ability to control the rotor speed. Faster rotor speed results in harder and more frequent blows, where as a slower speed allows for a tighter particle size distribution.

4. Is the lab scale hammer mill scalable?

Absolutely. The lab scale hammer mill is, quite simply a miniature version of larger production models. Thus, the lab scale hammer mill production results can be extrapolated to that of its larger counterparts.

5. What can I expect in terms of set-up and maintenance?

The lab scale hammer mill is fully assembled upon delivery. The operator simply needs to connect the mill's motor or control panel to the main building power source. The top of the hammer mill hinges open to allow easy access for cleaning, screen changes and hammer rotation. As with larger hammer mills, optional dust collection systems can be integrated if needed.